The present invention relates generally to a transmission interface, and particularly to a transmission interface for a non-industry standard bus that is compatible with the AT Attachment Packet Interface""s Task File.
Satisfying the apparently insatiable demand for ever-increasing microprocessor clock rates presents a challenge to designers of Compact Disc Read-Only-Memory (CD-ROM) devices. FIG. 1 illustrates the cause of this challenge, showing a CD-ROM Device 30 connected to a personal computer (PC) 32 via an Integrated Disc Electronics (IDE) Bus 34. Consisting of 40 wires, IDE Bus 34 has a maximum clock rate of 66 MHZ and supports up to 32 bits of parallel data. IDE Bus 34 transports single-ended, parallel signals that use Transistor-to-Transistor Logic (TTL) voltage level signaling. In other words, each line, or wire, of IDE Bus 34 carries a single signal that represents a digital xe2x80x9c1xe2x80x9d via a voltage level of approximately 5 volts and a digital xe2x80x9c0xe2x80x9d via a voltage level of approximately 0 volts.
AT Attachment (ATA) Interface 36a enables PC 30 to support CD-ROM players. ATA Interface 36a is coupled to the microprocessor""s local bus, Peripheral Component Interconnect (PCI) 40. The maximum clock rate of IDE Bus 34 is limited by that of PCI 40; i.e., 66 MHZ. This clock rate is not adequate to enable PC 32 to simultaneously play music and video stored on a CD-ROM. Thus, the demand for speed militates that data transfer rates between CD-ROM player 30 at least equal, if not exceed, the clock rate of PCI 40.
One solution is to increase the width of the data path between PC 32 and CD-ROM Device 30, i.e., increasing the number of lines of IDE Bus 34. A transfer rate of greater than 66 MHZ could be achieved by doubling the number of wires of IDE Bus 34 from 40 to 80; however, such a large pin/wire count is unlikely to gain wide acceptance. Another approach to achieving higher clock and data transfer rates would be to couple CD-ROM Device 30 to Direct Memory Access (DMA) Interface 42, rather than PCI 40. Increasing the data transfer rate in this manner comes at the cost of backward compatibility with devices using ATA Interface 36. Thus, a need exists for an interface that supports data transfer rates greater than that possible with the IDE Bus 34, is compatible with the ATA Interface and uses no more than the number of wires of IDE Bus 34.
No technology currently available entirely satisfies this need. IEEE Standard 1394 defines a high speed, isochronous, external bus for personal computers. Sometimes called a xe2x80x9cFire Wirexe2x80x9d because of its speed, the 1394 bus is not widely used, despite its speed and flexibility, because of its expense.
AT Attachment Packet Interface (ATAPI) for CD-ROMs is an extension of the ATA Interface that supports connection of CD-ROM players and tape players to personal computers. The ATAPI Standard (SFF-8020i) defines a Task File, a set of registers used by the peripheral devices and personal computer, used to transfer data. According to ATAPI, commands are communicated using packets. Generally described, a packet is a portion of a message, which may include many packets. Typically, each packet includes destination information and data, or a payload. A packet may also include a packet ID (PID), data, which forms the packet payload, and a cyclical redundancy check (CRC). Because each packet of a message includes a PID, packets need not be transmitted in order to successfully reconstruct the message. Many protocols using packets support isochronous data transfer, as opposed to synchronous data transfer. Isochronous data transfer enables video data to be transmitted as quickly as it is displayed and generally supports very high data transfer rates. However, devices using ATAPI also typically use the IDE Bus, thereby limiting the maximum data transfer rate below the theoretical maximum rate.
Low Voltage Differential Signaling (LVDS) is an alternative to standard signaling, which uses TTL voltage levels and is single-ended. LVDS data transmission is less susceptible to common-mode noise than a single-ended scheme because two wires with opposite current/voltage swings are used instead of a single wire. Because of the reduced noise concerns, low voltage level swings can be used thereby reducing power consumption and allowing faster switching rates. However, merely replacing each single-ended wire of the IDE bus with two LVDS wires is unacceptable because of the increased wire count of the resultant bus as compared to the IDE bus.
The present invention is a transmission interface compatible with the ATA Interface and the AT Attachment Packet Interface (ATAPI) that achieves transfer rates greater than those possible with the IDE bus by using serial, low voltage differential signaling (LVDS). The transmission interface of the present invention includes a transmission ATAPI circuit, a packetizing circuit and a converter. The transmission ATAPI circuit monitors the content of the ATAPI and, when a change is detected, generates a first set of signals representative of that change. The first set of signals are single-ended, parallel to one another and use Transistor-Transistor Logic (TTL) voltage levels. The packetizing circuit packetizes the first set of signals to generate a second set of signals, which represent a packet. The packet payload represents the change in the contents of the ATAPI. The second set of signals are also single-ended, parallel to one another and use TTL voltage levels. The converter converts the second set of signals into a third set of signals and couples these to a serial bus. The third set of signals are serial to one another, and use low voltage level, differential signaling. Thus, the third set of signals are suited for transmission by the serial bus, which includes many fewer wires than available in an IDE bus while operating at a faster data rate.